fix: Remove shared mutable state between database instances

[AndrewSisley]

Relevant issue(s)

Resolves #2774

Description

Removes shared mutable GQL state between database instances.

These global static GQL type variables are actually mutated by the GQL library we use at different points during database init, and schema update. This means that when multiple Defra instances are created in the same process they were sharing state. As well as just being an inherently bad thing, this also prevented us from running tests within packages in parallel using t.Parallel() as the race detector would complain.

Our integration tests now can all run with t.Parallel() and the --race flag (tested locally). I do not wish to make that change within this PR though for cleanliness/focus reasons. The --race flag is still flaky in the CI, this PR improves things but does not let us Parellize the tests fully. I believe that the shared state in https://github.com/graphql-go/graphql/blob/master/introspection.go is responsible for the remaining failures, I'd rather spend time moving away from the package entirely than to rework that, however I still think this PR represents an improvement and would like to merge.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.58%. Comparing base (5855969) to head (7b55fb8).

❗ Current head 7b55fb8 differs from pull request most recent head 92aa500

Please upload reports for the commit 92aa500 to get more accurate results.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2777      +/-   ##
===========================================
+ Coverage    78.19%   78.58%   +0.40%     
===========================================
  Files          313      315       +2     
  Lines        23116    23810     +694     
===========================================
+ Hits         18074    18711     +637     
- Misses        3667     3719      +52     
- Partials      1375     1380       +5     

Flag	Coverage Δ
all-tests	78.58% <100.00%> (+0.40%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
internal/request/graphql/schema/collection.go	90.33% <100.00%> (ø)
internal/request/graphql/schema/generate.go	87.45% <100.00%> (+0.02%)	⬆️
internal/request/graphql/schema/manager.go	98.13% <100.00%> (+0.30%)	⬆️
internal/request/graphql/schema/types/base.go	100.00% <100.00%> (ø)
internal/request/graphql/schema/types/commits.go	100.00% <100.00%> (ø)
internal/request/graphql/schema/types/scalars.go	100.00% <100.00%> (ø)
internal/request/graphql/schema/types/types.go	100.00% <100.00%> (ø)

... and 21 files with indirect coverage changes

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[fredcarle]

LGTM